Pyrolysis is a thermal process during which solid carbonaceous biomass feedstock, i.e., “biomass”, such as wood, agricultural wastes/residues, algae, forestry byproducts, cellulose and lignin, municipal waste, construction/demolition debris, or the like, is rapidly heated to pyrolysis temperatures of about 300° C. to about 900° C. in the absence of air using a pyrolysis reactor. A modern form of pyrolysis, or rapid thermal conversion, is conducted under moderate temperatures, typically 400 to 600° C., and short residence times of less than 5 seconds. An example is RTP or Rapid Thermal Processing that operates under such conditions producing solid and gaseous pyrolysis products. The gaseous pyrolysis products (“pyrolysis gases”) comprise a non-condensable portion and a condensable portion (vapors) that can be condensed into liquid biomass-derived pyrolysis oil. The solid pyrolysis products include combustible solids containing carbon, referred to as “char”.
Heat for the endothermic pyrolysis reaction is produced in a combustion zone of the process by combusting the char or by combusting the char and the non-condensable pyrolysis gases in the presence of the heat transfer medium. Heat is transferred from the reheater to the pyrolysis reactor by the “heat transfer medium.” The heat transfer medium typically comprises inert solids, such as silica sand, low activity catalyst, or other inert material.
In such processes, the heat transfer medium and the solid fuel are typically separated from the gaseous products by a momentum device, such as a cyclonic separator.
However, a portion of the circulating heat transfer medium and the char are too small in diameter to be separated by a momentum device. As a result, after condensation of the liquid pyrolysis oil, the liquid fuel contains some solids from the sand and char that were not separated by the momentum separator. In addition, the char contains some metals from the biomass feedstock, such as sodium, potassium, calcium, and magnesium. These metals contribute to the instability of the liquid fuel in storage. The viscosity increases over time, eventually leading to a separation of the fuel into an organic phase and an aqueous phase. Although possible, it is more difficult to use this high viscosity oil as fuel. The presence of solids in the oil can cause problems. In addition, the oil has to be heated to be used because of the high viscosity, which can cause the oil to solidify and/or accelerate corrosion of the fuel system.
Filtration has been used for separation, either alone or in combination with a momentum separator, to generate a lower-solids content liquid fuel. However, the additional residence time introduced by such filters causes some of the liquid fuel to be converted into gaseous fuel. See, e.g., Kang, et al., Fast pyrolysis of radiate pine in a bench scale plant with a fluidized bed: Influence of a char separation system and reaction conditions on the production of bio-oil, J. Anal. Appl. Pyrolysis 76 (2006) p. 32-37; and Park et al., Pyrolysis characteristics of Oriental white oak: Kinetic study and fast pyrolysis in a fluidized bed with an improved reaction system, Fuel Processing Technology 90 (2009) 186-195.